Field of the Invention
The present invention relates to an inorganic polarizing plate, and a production method thereof.
Description of the Related Art
As in a liquid crystal display device, it is necessary to provide a polarizing plate at a surface of a liquid crystal panel due to image formation principles thereof. The functions of the polarizing plate is to absorb one of the polarizing components (i.e., p-waves, and s-waves) orthogonal to each other, and passing through the other component.
Conventionally, a dichroic polarizing plate containing an iodine-based or dye-based polymeric organic material within a film has been widely used as the aforementioned polarizing plate. As for a typical production method of such the polarizing plate, used is a material containing dying a polyvinyl alcohol-based film with a dichroic material, such as iodine, crosslinking using a crosslinking agent, and performing uniaxial drawing. As the dichroic polarizing plate is produced by drawing as mentioned above, the dichroic polarizing plate is typically easily shrunk. Moreover, a hydrophilic polymer is used in the polyvinyl alcohol-based film, therefore, it is extremely easily deformed, particularly under humid conditions. Moreover, the polarizing plate fundamentally uses a film, and hence a mechanical strength as a device is weak. In order to avoid this, a method for bonding a transparent protective film to the polarizing plate has been used.
Recently, use of liquid crystal display devices has been increased, and performances thereof have been improved. Along with this trend, high reliability and durability have been demanded for each device constituting a liquid crystal display device. In case of a liquid crystal display device using a light source of a largo high radiation intensity, such as a transmissive liquid crystal projector, for example, a polarizing plate receives strong radiant rays. Therefore, the polarizing plate for the aforementioned use requires excellent heat resistance. As the aforementioned film-based polarizing plate is organic matter, there are limits for improving the aforementioned properties.
In USA, an inorganic polarizing plate having high heat resistance has been on marked under the product name of Polarcor from Corning Incorporated. This polarizing plate has a structure where silver particles are scattered in glass, and does not use an organic material, such as a film. The principle thereof uses plasma resonance of island-state particles. Specifically, light absorption due to surface Plasmon resonance is utilized when light enters the island-state particles of rare metal or transmission metal, and the absorption wavelength is influenced by shapes of the particles, or a dielectric constant of the surroundings. When the shapes of the island-state particles are ovals, the resonance wavelength is difference between the major axis direction thereof, and the minor axis direction thereof. As a result, polarizing properties can be attained. Specifically, the polarizing properties that the polarizing component parallel to the major axis is absorbed at the side of the long wavelength side, and the polarizing component parallel to the minor axis is passed through can be attained. In case of Polarcor, however, the wavelength range with which the polarizing properties can be attained, is a region close to the infrared region, and does not cover a visible region required by a liquid crystal display device. It is assumed that this is because of physical characteristics of silver used for island-state particles.
U.S. Pat. No. 6,772,608 discloses an UV polarizing plate, in which particles are precipitated in glass by thermal reduction, using the aforementioned principle, and discloses that use of silver as a specific example of metal particles. In this case, it is assumed that the absorption at the minor axis direction is used, different from the aforementioned Polarcor. As depicted in FIG. 1 therein, the disclose polarizing plate functions as a polarizing plate even at the range adjacent to 400 nm, but the extinction ratio is small and the band of the light that can be absorbed is extremely narrow. Therefore, even by combining Polarcor and the technology disclosed in U.S. Pat. No. 6,772,608, a polarizing plate that can cover an entire visible range cannot be attained.
Moreover, a theoretical analysis of an inorganic polarizing plate using plasma resonance of island-sate metal particles is disclosed in J. Opt. Soc. Am. A, Vol. 8, No. 4, 619-624. According to this literature, a resonance wavelength of aluminium particles is shorter than that of silver particles by about 200 nm. It is disclosed that there is a possibility that a polarizing plate covering a visible range can be produced by using aluminium particles.
Moreover, Japanese Patent Application Laid-Open (JP-A) No. 2000-147253 discloses several production methods of a polarizing plate using, aluminium particles. In this literature, it is described that silicate-based glass is not desirable as a substrate, as aluminium and glass are reacted, and calcium.aluminoborate glass is suitable as a substrate (the paragraphs [0018], [0019]). However, glass using silicate is widely distributed as optical glass, and highly reliable products thereof can be available at low cost. Therefore, it is not economically preferable, if these products are not suitable for use as a substrate. Moreover, a method for forming island-state particles through etching a resist pattern is described therein (the paragraphs [0037], [0038]). Typically, a polarizing plate used in a projector needs a size of about several centimeters, and requires a high extinction ratio. In the case where a polarizing plate is intended to be a polarizing plate for visible light, therefore, a size of a resist pattern for use needs to be sufficiently shorter than visible light wavelengths, i.e., several tens nanometers. Moreover, a highly dense pattern needs to be formed to attain a high extinction ratio. In the case where the polarizing plate is used for a projector, moreover, a pattern of a large area needs to be formed. However, the method for applying the high-density fine pattern formation using the disclosed lithography needs use electron beam drawing in order to attain the pattern, as mentioned above. The electron beam drawing is a method for drawing each pattern with electron beams, productivity thereof is poor, and therefore it is not practical.
Moreover, JP-A No. 2000-147253 discloses that aluminium is removed by chlorine plasma. In the case where etching is performed in this manner, typically, chloride is deposited on side walls of the aluminium pattern. It is possible to remove the chloride with a commercially available etching solution (e.g., SST-A2 of Tokyo Ohka Kogyo Co., Ltd.), but such a chemical fluid, which reacts with aluminium chloride, also reacts with aluminium, although etching speed thereof is slow. Therefore, it is difficult to realize formation of a desired pattern using the disclosed method.
Furthermore, JP-A No. 2000-147253 discloses, as another method, a method containing depositing aluminium on a patterned photoresist through oblique film formation, and removing the photoresist (the paragraphs [0045], [0047]). However, it is assumed that it is necessary to deposit aluminium on a surface of a substrate to some degrees in order to attain adhesion between substrate and aluminium in this method. This means that a shape of the deposited aluminium film is different from a prolate sphere including a prolate oval, which is described as a suitable shape in the paragraph [0015]. Moreover, it is described in the paragraph [0047] that the excessive deposits are removed by anisotropic etching vertical to the surface. In order to function as a polarizing plate, anisotropy of a shape of the aluminium is important. Accordingly, it is assumed that it is necessary to adjust an amount of the aluminium deposited on the resist part and the substrate surface in order to attain a desired shape. However, it is considered that it is extremely difficult to control in a size of submicron or smaller, e.g., 0.05 μm, as described in the paragraph [0047]. Therefore, doubt remains whether or not this method is suitable as a production method of high productivity. As for properties of a polarizing plate, high transmittance is required in a transmission axial direction. In the case where glass is used as a substrate, typically, a few percent of reflection at an interlace of the glass cannot be avoided, and hence it is difficult to attain high transmittance.
Moreover, JP-A No. 2002-372620 discloses a polarizing plate formed through oblique deposition. This method is to attain polarizing properties by producing a fine prismatic structure through oblique deposition of material that is transparent or opaque to wavelengths of light in a wavelength range for use, and is a method having high productivity, as a fine pattern can be attained with a simple method, different from JP-A No. 2000-147253. However, this method also has a problem. An aspect ratio of the formed fine prismatic structure of the material transparent or opaque to a wavelength range for use, a pitch of individual prisms in the fine prismatic structure, and a linearity are important elements for attaining excellent polarizing properties, and these should be intentionally controlled in view of reproducibility of the properties. In this method, however, used is a phenomenon that a prismatic structure is obtained, as depositing particles are not deposited in shadow areas of an initial deposition layer formed by previously deposited particles. Therefore, it is difficult to intentionally control the aforementioned items. As a method for improving this problem, a method for providing polishing traces on a substrate by a rubbing process before the deposition is described. However, diameters of particles constituting a deposition film are typically about several ten nanometers, and it is necessary to intentionally produce a pitch of submicron or smaller through polishing, in order to control anisotropy of these particles. When a typical polishing sheet is used, a size of about submicron is the limit. Therefore, it is not easy to produce such fine polishing traces. As described earlier, moreover, a resonance wavelength of Al particles largely depends on a refractive index of the surroundings. In this case, it is importance to combine a transparent material and an opaque material. In JP-A No. 2002-372620, a combination for attaining excellent polarizing properties in a visible region is not disclosed. In the case where glass is used as a substrate, similarly to JP-A No. 2000-147253, reflectance of several percent at an interface of the glass cannot be avoided.
Moreover, Applied Optics, Vol. 25, No. 21986 311-314 discloses a polarizing plate for infrared transmission, which is called Lamipol. This polarizing plate has a laminate structure of Al and SiO2. According to the literature, the polarizing plate exhibits an extremely high extinction ratio. Moreover, J. Lightwave Tec., Vol. 15, No. 6, 1997, 1042-1050 discloses that a high extinction ratio with a wavelength of 1 μm or shorter can be realized by using Ge instead of Al of Lamipol, which is configured to absorb light. From FIG. 3 of this literature, it is expected that a high extinction ratio can be also attained using tellurium (Te). As described above, Lamipol is an absorbing polarizing plate with which a high extinction ratio can be attained. However, it is not suitable for a polarizing plate for a projector, which requires a size of a several centimeter in each side, as a thickness of a laminate of a light-absorbing material, and a transmissive material becomes a size of a light-accepting surface.
Furthermore, JP-A No. 2008-216957 discloses a polarizing plate combining a wire grid structure, and an absorbing film. In the case where a metal or semiconductor film is used as the absorbing film, it is largely influenced by optical properties of the material. Therefore, it is possible to reduce reflectance of light in a certain region by adjusting film thicknesses of dielectric materials between the material, the wire grid, and the absorbing film. However, it is difficult to realize the reduction of reflectance in a wider wavelength range.
Moreover, it is possible to widen the bandwidth by using Ta or Ge having high absorbance. However, the absorbance in the transmission axis direction also increases, which reduces the transmittance in the transmission axis direction, which is one of important properties of the polarizing plate.
Moreover, JP-A No. 2011-113631 and JP-A No. 2003-508813 each disclose an inorganic polarizing plate, in which a cross-sectional shape of a metal wire in a wire grid layer is a trapezoid. In the proposed technologies, however, polarizing properties are not sufficient.
Accordingly, there currently needs for providing an inorganic polarizing plate having excellent polarizing properties, and a production method thereof.